Frequency-veering and mode hybridization in arch bridges

The assessment of the structural conditions of existing structures and the evaluation of their seismic performances are usually conducted by means of numerical models. Differently than in the case of a new structure when the construction following the design phase should be done trying to reproduce, in the best way, the design assumptions, assessment analysis requires the determination of a numerical model able to accurately reproduce the actual behavior of the existing construction. This latter issue leads to an inverse problem, whose difficulty is always higher than the direct design problem. In this paper, model updating strategies based on dynamic data are applied to a class of six reinforced concrete twin arch bridges located in Provincia di Teramo, Italy, to assess their structural performance under seismic actions. The attention will be mainly focused on the Valle Castellana bridge. The case study has been selected because an apparently strange dynamic behavior emerged in a dynamic campaign carried out on 2002–2003 on this bridge. The peculiar behavior can be linked to a known phenomenon possibly affecting initially curved, symmetric structures, when the initial symmetry is lost for different possible reasons. In addition, on 2009, the Valle Castellana bridge suffered for a sudden settlement of one of the supports, and restoration works were needed to reconstruct the parts subject to failure. Based on the results of dynamic tests carried out on 2011, a calibrated finite element model was determined and used to assess the actual structural behavior of the bridge. A possible structural improvement of its seismic performance based on the validated numerical model is also discussed.